arXiv subjects

Natchimuthuk Gopalswamy

Publications and source records attributed to Natchimuthuk Gopalswamy.

5 recordsLinked to original sources

arXiv2025

Role of CME clusters and CME-CME interactions in producing sustained $γ$-ray emission

Fast (V$_{\rm CME}$>1000${\rm \,km\,s^{-1}}$) coronal mass ejections (CMEs) capable of accelerating protons beyond 300MeV are thought to trigger hours-long sustained $γ$-ray emission (SGRE) after the impulsive flare phase. Meanwhile, CME-CME interactions can cause enhanced proton acceleration, increasing the fluxes of solar energetic particles. This study explores the role of fast CME interactions in SGRE production during CME clusters, which we define as a series of CMEs linked to >C-class flares with waiting times <$\,$1$\,$day from the same active region (AR). We focus on clusters in major CME-productive ARs (major ARs), by defining a major AR as one that produced >$\,$1 CME-associated major (>M-class) flare. The study identified 76 major ARs between 2011 and 2019, of which 12 produced all SGRE events. SGRE-producing ARs exhibit higher median values for the speed of their fastest CMEs (2013 vs. 775${\rm \,km\,s^{-1}}$) and the class of their strongest flares (X1.8 vs. M5.8), compared to SGRE-lacking ARs. They also produced relatively faster CMEs (median speed: 1418 vs. 1206.5${\rm \,km\,s^{-1}}$), with the SGRE-associated CMEs occurring during periods of higher CME rates than typical fast CME epochs. Twelve of 22 (54.5%) SGRE events and 5 of 7 (71.4%) long-duration (>$10\,$h) SGRE events occurred during CME clusters, with high chances of CME-CME interactions. A case study on very active major ARs showed that all SGRE-associated CMEs with V$_{\rm CME}\lesssim$ 2000${\rm \,km\,s^{-1}}$ underwent CME-CME interactions within 10$\,$R$_\odot$, while SGRE-associated CMEs faster than 3000${\rm \,km\,s^{-1}}$ did not undergo interactions.

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arXiv2025

Modeling Polarized Radio Sounding Observations of a Coronal Mass Ejection

Coronal Mass Ejections (CMEs) evolve significantly as they propagate from the Sun to the Earth, so remote observations of their changes in speed, strength of the magnetic field, density, and overall structure are critical for predicting their arrival time and geoeffectiveness. Radio line-of-sight observations of Faraday rotation and Total Electron Content combined with white-light observations enables the measurement of these properties with careful analyses. This paper describes the analysis techniques and evaluates their accuracy with regard to measuring a CME's complex evolving structure and speed. The approach utilizes the layout of the Faraday effect tracker of coronal and heliospheric structures (FETCH), a purely space-based instrument concept, with Alfven Wave Solar atmosphere Model (AWSoM) simulations as input for evaluating these radio-based measures. Focusing on density and velocity/speed, we find that in-situ measurements of CME properties observe similar but different aspects of the distinct CME structure. The AWSoM model suggests that Faraday rotation may be a more sensitive measure of structure than Total Electron Content (TEC). Finally, we discuss the difficulty the simulation reveals in determining the trailing edge location of a magnetic flux rope.

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arXiv2025

Results from NASA's First Radio Telescope on the Moon: Terrestrial Technosignatures and the Low-Frequency Galactic Background Observed by ROLSES-1 Onboard the Odysseus Lander

Radiowave Observations on the Lunar Surface of the photo-Electron Sheath instrument (ROLSES- 1) onboard the Intuitive Machines' Odysseus lunar lander represents NASA's first radio telescope on the Moon, and the first United States spacecraft landing on the lunar surface in five decades. Despite a host of challenges, ROLSES-1 managed to collect a small amount of data over fractions of one day during cruise phase and two days on the lunar surface with four monopole stacer antennas that were in a non-ideal deployment. All antennas recorded shortwave radio transmissions breaking through the Earth's ionosphere -- or terrestrial technosignatures -- from spectral and raw waveform data. These technosignatures appear to be modulated by density fluctuations in the Earth's ionosphere and could be used as markers when searching for extraterrestrial intelligence from habitable exoplanets. After data reduction and marshaling a host of statistical and sampling techniques, five minutes of raw waveforms from the least noisy antenna were used to generate covariances constraining both the antenna parameters and the amplitude of the low-frequency isotropic galactic spectrum. ROLSES- 2 and LuSEE-Night, both lunar radio telescopes launching later in the decade, will have significant upgrades from ROLSES-1 and will be set to take unprecedented measurements of the low-frequency sky, lunar surface, and constrain the cosmological 21-cm signal. ROLSES-1 represents a trailblazer for lunar radio telescopes, and many of the statistical tools and data reduction techniques presented in this work will be invaluable for upcoming lunar radio telescope missions.

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arXiv2024

Novel scaling laws to derive spatially resolved flare and CME parameters from sun-as-a-star observables

Coronal mass ejections (CMEs) are often associated with X-ray (SXR) flares powered by magnetic reconnection in the low-corona, while the CME shocks in the upper corona and interplanetary (IP) space accelerate electrons often producing the type-II radio bursts. The CME and the reconnection event are part of the same energy release process as highlighted by the correlation between reconnection flux ($ϕ_{rec}$) that quantifies the strength of the released magnetic free energy during SXR flare, and the CME kinetic energy that drives the IP shocks leading to type-II bursts. Unlike the sun, these physical parameters cannot be directly inferred in stellar observations. Hence, scaling laws between unresolved sun-as-a-star observables, namely SXR luminosity ($L_X$) and type-II luminosity ($L_R$), and the physical properties of the associated dynamical events are crucial. Such scaling laws also provide insights into the interconnections between the particle acceleration processes across low-corona to IP space during solar-stellar 'flare- CME- type-II' events. Using long-term solar data in SXR to radio waveband, we derive a scaling law between two novel power metrics for the flare and CME-associated processes. The metrics of 'flare power' ($P_{flare}=\sqrt{L_Xϕ_{rec}}$) and 'CME power' ($P_{CME}= \sqrt{L_R {V_{CME}}^2}$), where $V_{CME}$ is the CME speed, scale as $P_{flare}\propto P_{CME}^{0.76 \pm 0.04}$. Besides, $L_X$ and $ϕ_{rec}$ show power-law trends with $P_{CME}$ with indices of 1.12$\pm$0.05 and 0.61$\pm$0.05 respectively. These power-laws help infer the spatially resolved physical parameters, $V_{CME}$ and $ϕ_{rec}$, from disk-averaged observables, $L_X$ and $L_R$ during solar-stellar 'flare- CME- type-II' events.

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arXiv2024

Energetic particle activity in AD Leo: Detection of a solar-like type-IV burst

AD Leo is a young and active M dwarf with high flaring rates across the X-ray to radio bands. Flares accelerate particles in the outer coronal layers and often impact exospace weather. Wide-band radio dynamic spectra let us explore the evolution of particle acceleration activity across the corona. Identifying the emission features and modelling the mechanisms can provide insights into the possible physical scenarios driving the particle acceleration processes. We performed an 8 h monitoring of AD Leo across the 550 - 850 MHz band using upgraded-Giant Metrewave Radio Telescope (uGMRT). A python routine, named VISAD, was built to obtain the visibility averaged wide-band dynamic spectra. Direct imaging was also performed. Based on existing observational results on AD Leo and on solar active region models, radial profiles of electron density and magnetic fields were derived. Applying these models, we explore the possible emission mechanisms and magnetic field structure of the active region. The star displayed high brightness temperature ($\approx 10^{10} - 10^{11}$ K) throughout the observation with nearly 100% left circularly polarised bursts. Post flare phase is characterised by a highly polarised (60 - 80%) solar-like type IV burst confined above 700 MHz.The flare emission favors a Z-mode or a higher harmonic X-mode electron cyclotron maser emission mechanism. The post-flare activity above 700 MHz is consistent with a type-IV radio burst from flare-accelerated particles trapped in magnetic loops, which could be a coronal mass ejection (CME) signature. This is the first solar-like type-IV burst reported on a young active M dwarf belonging to a different age - related activity compared to the Sun. We also find that, a multipole expansion model of the active region magnetic field better accounts for the observed radio emission than a solar-like active region profile.

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